Field of the Invention
The present invention relates generally to corrosion-resistant metal tubing and, more particularly, to nickel-iron-chromium alloys that are particularly useful in corrosive oil and gas well environments where high strength, corrosion resistance and reasonable cost are desired attributes.
Description of Related Art
As older shallow and less corrosive oil and gas wells are depleted, higher strength and more corrosion-resistant materials are needed to allow for deeper drilling which encounters more corrosive environments.
Oil patch applications now require alloys of increasing corrosion resistance and strength. These increasing demands arise from factors including: deep wells that involve higher temperatures and pressures; enhanced recovery methods such as steam or carbon dioxide (CO2) injection; increased tube stresses especially offshore; and corrosive well constituents including hydrogen sulfide (H2S), CO2 and chlorides.
Materials selection is especially critical for sour gas wells—those containing H2S. Sour well environments are highly toxic and extremely corrosive to traditional carbon steel oil and gas alloys. In some sour environments, corrosion can be controlled by using inhibitors along with carbon steel tubulars. The inhibitors, however, involve continuing high cost and are often unreliable at high temperatures. Adding corrosion allowance to the tubing wall increases weight and reduces interior tube dimensions. In many cases, the preferred alternative in terms of life-cycle economy and safety is the use of a corrosion-resistant alloy for tubulars and other well components. These corrosion-resistant alloys eliminate inhibitors, lower weight, improve safety, eliminate or minimize workovers and reduce downtime.
Martensitic stainless steels, such as the 13% chromium alloys, satisfy corrosion resistance and strength requirements in slightly corrosive oil patch applications. The 13% alloys, however, lack the moderate corrosion resistance and strength required of low-level sour gas wells. Cayard et al., in “Serviceability of 13Cr Tubulars in Oil and Gas Production Environments”, published sulfide stress corrosion data that indicate 13Cr alloys have insufficient corrosion resistance for wells that operate in the transition region between sour gas and non-sour gas environments. Further background art may be found in U.S. Pat. No. 4,358,511 to Smith, Jr. et al. and U.S. Pat. No. 5,945,067 to Hibner et al.
While the mildly corrosive wells are handled by various 13Cr steels, Ni-base alloys are needed for the more highly corrosive environments. Among the more commonly used Ni-base alloys for oil patch use are austenite high-Ni-base alloys such as, for example, alloys 718, 725, 825, 925, G-3 and C-276, which provide increased resistance to corrosive sour gas environments. These aforementioned alloys, however, are either too expensive or do not possess the necessary combination of high strength and corrosion resistance.
U.S. Pat. No. 7,416,618 to Mannan et al. discloses nickel-iron-chromium alloys formed by annealing and age hardening. However, tubing manufactured according to the process has not satisfied all material requirements for manufacturing of tubing meeting current aims in oil and gas exploration and drilling applications.
Huizinga et al., in “Offshore Nickel Tubing Hanger and Duplex Stainless Steel Piping Failure Investigations”, discloses that several prominent oil and gas failures of alloy 718 exploration and drilling components have raised legitimate toughness and microstructure concerns of precipitated-hardened alloys in field service. In the case of alloy 718, the microstructural feature causing cracking was identified as delta phase (Ni3Cb). Cassagne et al, in “Understanding Field Failures of Alloy 718 Forging Materials in HP/HT wells”, has suggested that hydrogen embrittlement is promoted by any inter-granular second phase irrespective of chemical composition. Mannan et al. in “Physical Metallurgy of Alloys 718, 725, 725HS, 925 for Service in Aggressive Corrosive Environments”, has shown that the presence of significant amounts of any second phase lowers time-to-failure, % elongation and reduction-of-area ratios in SSR (slow strain rate) tests. Further, it degrades tensile reduction-in-area and impact strength. These observations have resulted in the requirement that such alloys, to be certified for oil and gas field applications, must possess a clean microstructure and minimum impact strength in addition to the usual required properties needed for any given application. The American Petroleum Institute (API) Specification of Nickel Base Alloy 718 (UNS N07718) sets acceptance criteria for metallographic examination for deleterious phases for Nickel Base Alloy 718.
The present invention solves the problems encountered in the prior art by providing a tubing and process of manufacturing thereof that satisfies current industry requirements for use in oil and gas completion and drilling applications.